This invention relates to photovoltaic generators for producing a control signal for the control of a semiconductor switching device, and more specifically relates to a novel structure for such a device in which an array of planar cells formed in a silicon substrate are dielectrically isolated from one another in an insulated wafer-bonded assembly.
Photovoltaic generators (PVG) are well known and are commonly used for producing a control signal for a solid state relay. Such devices employ an LED which is energized by input terminals to irradiate the photosensitive surface of a spaced and insulated photovoltaic device. The output of the photovoltaic device may serve as the input to a switching device, such as a MOS-gated device, typically a power MOSFET, which has load terminals which are switched "on" in response to the energization of the LED. The input and output terminals of the relay are isolated by the gap between the LED and the photovoltaic device. Commonly, the photovoltaic device consists of a large number of series-connected photovoltaic cells in order to produce a voltage sufficiently high to turn on the power switching device. Such devices are well known and are sold under the name "PVI" (photovoltaic isolator) by the International Rectifier Corporation of El Segundo, Calif., the assignee of the present invention.
The plural cell photogenerator can be made in many different ways. One known generator employs a stack or pile of photovoltaic cells as shown in U.S. Pat. No. 4,755,697 to Kinzer. Other devices employ a planar array of cells which are junction isolated from one another and are connected in series at their surfaces. Still other devices are known in which individual cells disposed over the surface of a silicon chip are junction-isolated from one another or may be dielectrically isolated, as shown in U.S. Pat. Nos. 4,227,098 and 4,390,790.
The prior art devices have the drawback of being expensive to manufacture and having low manufacturing yields. It is desirable to produce a photovoltaic generator which is capable of producing a large number of insulated cells which can be connected in series to produce a turn-on signal for a power MOS-gated device but which is easily manufactured, using existing reliable processing equipment and techniques.